The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. In its waters are abundant marine life but the environment is changing. The shells of young oysters in Chesapeake Bay are not getting as thick as they've been in the past, and higher acidity levels seem to be to blame.

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The bay is mostly known for its great seafood production, especially blue crabs, clams and oysters. The plentiful oyster harvests led to the development of the skipjack, the state boat of Maryland, which is the only remaining working boat type in the United States still under sail power.

Today, the body of water is less productive than it used to be, because of runoff from urban areas (mostly on the Western Shore) and farms (especially on the Eastern Shore), over harvesting, and invasion of foreign species. The bay still yields more fish and shellfish (about 45,000 tons annually) than any other estuary in the United States.

Oysters provide a natural way of filtering the bay waters of all sorts of suspended debris. At one time the local oyster population was huge but in the present era it is only at a fraction of what it once was. There has been some efforts to improve the situation and oyster production is up. However, scientists have found a potential problem.

"The regional changes in acidity revealed in our analysis are greater than what could be caused by increasing atmospheric carbon dioxide alone," says George Waldbusser, the lead author of a study by the University of Maryland Center for Environmental Science that looked at 23 years of water quality data.

"We are seeing a complex pattern of increasing acidity in the more saline regions of the Bay, but the opposite trend of decreasing acidity in the less saline waters of the Bay," he says.

Other research has shown that thinner shells make juvenile oysters more vulnerable to crabs, which will feed on them.

As nutrients from sewage systems and farm runoff go into the waters, they promote more phytoplankton populations in the upper Chesapeake Bay. The plants grow and absorb large amounts of carbon dioxide, making the waters less acidic. Scientists say as the phytoplankton are carried toward the ocean, animals and bacteria consume them. The animals and bacteria then release the same carbon dioxide taken up by the phytoplankton. It remains dissolved in the water making the water significantly more acidic, researchers suggest.

"Carbon dioxide is a weak acid and whether it is produced by fossil fuel combustion or enhanced phytoplankton growth and decay, the resulting acidic conditions have the same negative effect on shell forming animals, such as oysters and corals," Waldbusser says.

What we have is not just one cause for the acidity but more how the entire bay and estuary ecosystem is responding to changing environmental conditions.